Dark Matter Crisis

Pavel Kroupa invited me to comment on his blog about a comment left asserting that it was absurd to question dark matter. In the interest of full disclosure, I provide here the full text that I suggested:

What is absurd is to presume that a theory that is adequate on one scale will inevitably succeed on another scale. That ΛCDM is consistent with large scale measurements is no guarantee that it will work on smaller scales. The widespread presumption among cosmologists seems to be that it must surely work out, though this is hardly a scientific attitude.

The failure of dark matter to explain galaxy scale phenomena is not just a failure of detail. A single effective force (modified gravity) suffices to explain (and indeed, predicted a priori) many aspects of galaxy dynamics. To explain this with dark matter is like asserting that really the solar system operates on an inverse cube law; there just happens to be dark matter arranged just so as to always make it look like an inverse square law. It is hard to imagine a more bizarre requirement for the distribution of dark matter.

So far, all we know for certain is that there are mass discrepancies in extragalactic systems. The application of Newton's Laws to the observed distribution of baryons fails. This could be due either to dark matter or to a modification of the laws of motion on the appropriate scales. Since both hypotheses have demonstrated virtues, both remain logical possibilities. Until we detect dark matter in the laboratory, we cannot legitimately claim to be sure it exists.

28 July 2010